1. Field
The present invention relates generally to communications, and more specifically, to adaptive rate prioritizing in a variable rate communications system.
2. Background
Wireless communication systems are designed to allow multiple users to share a common communications medium. One such wireless communications system is a code division multiple access (CDMA) system. The CDMA communications system is a modulation and multiple access scheme based on spread-spectrum communications. In a CDMA communications system, a large number of signals share the same frequency spectrum and, as a result, provide an increase in user capacity. This is achieved by transmitting each signal with a different code that modulates a carrier, thereby spreading the signal over the entire spectrum. The transmitted signals can be separated in the receiver by a demodulator using a corresponding code to de-spread the desired signal. The undesired signals, whose codes do not match, contribute only to noise.
Originally developed for voice applications, CDMA communication systems today can support high data rate services. In these systems, a variable data rate request scheme may be used to communicate at the maximum data rate that can be supported by the current channel conditions. By way of example, a wireless device may be configured to measure the quality of a signal transmission from a base station, and provide feedback to the base station to control the data rate.
In most high data rate CDMA communication systems today, the data rates available to the-base station are predetermined. Each available data rate may be assigned to at least one rate index which is generally referred to in the art as a data rate control (DRC) index. Associated with each DRC index is a transmission format which includes a packet length for the payload. The packet length is generally defined in terms of time slots. A specific data rate may be assigned to more than one DRC index with different slot formats. In this case, the conventional approach has been to select the long slot format to increase the probability of potential throughput gain (i.e., ARQ early termination gain) that may occur if the wireless device is able to decode the payload before all the time slots have been transmitted. However, consistent preference for the long slot format has the drawback of reducing overall packing efficiency of the payload. Conversely, maximizing packing efficiency by consistently preferring a short slot format reduces the potential for ARQ early termination gain. Accordingly, it would be advantageous to provide a communications system with adaptable preference capability that enables adaptive selection between long and short slot formats for a given data rate without diminishing the ARQ gain.